What was the most important effect of european
nationalism in the 1800s?

The most important effect of European nationalism in the 1800s was that it led to wide-spread militarism which eventually led to warfare, since every country started to view the world through an "us vs. them" type of lens. 

In the late fifteenth century, a major world power that established colonies in the era of old imperialism was france or germany or spain. their colonial activity was mainly limited to america or africa or asia.